12to11/subsurface.c

/* Wayland compositor running on top of an X server.

Copyright (C) 2022 to various contributors.

This file is part of 12to11.

12to11 is free software: you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation, either version 3 of the License, or (at your
option) any later version.

12to11 is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with 12to11.  If not, see <https://www.gnu.org/licenses/>.  */

#include <string.h>
#include <stdlib.h>

#include "compositor.h"

enum
  {
    PendingPosition	 = 1,
  };

enum _SurfaceActionType
  {
    Sentinel,
    PlaceAboveOther,
    PlaceBelowOther,
  };

typedef enum _SurfaceActionType SurfaceActionType;
typedef struct _Subsurface Subsurface;
typedef struct _Substate Substate;
typedef struct _SurfaceAction SurfaceAction;
typedef struct _SurfaceActionClientData SurfaceActionClientData;

#define SubsurfaceFromRole(role) ((Subsurface *) (role))

struct _SurfaceAction
{
  /* What this action is.  */
  SurfaceActionType type;

  /* What subsurface this action applies to.  */
  Subsurface *subsurface;

  /* What surface is the "other" surface.  */
  Surface *other;

  /* Surface destroy listener.  */
  DestroyCallback *destroy_listener;

  /* The next and last surface actions in this list.  */
  SurfaceAction *next, *last;
};

struct _Substate
{
  /* The position of the subsurface relative to the parent.  */
  int x, y;

  /* Various flags.  */
  int flags;
};

struct _Subsurface
{
  /* The role object itself.  */
  Role role;

  /* The parent surface.  */
  Surface *parent;

  /* The number of references to this subsurface.  */
  int refcount;

  /* Pending substate.  */
  Substate pending_substate;

  /* Current substate.  */
  Substate current_substate;

  /* Commit callback attached to the parent.  */
  CommitCallback *commit_callback;

  /* Whether or not this is synchronous.  */
  Bool synchronous;

  /* Whether or not a commit is pending.  */
  Bool pending_commit;

  /* The last dimensions and position that were used to update this
     surface's outputs.  */
  int output_x, output_y, output_width, output_height;
};

struct _SurfaceActionClientData
{
  /* Any pending subsurface actions.  */
  SurfaceAction actions;
};

/* The global wl_subcompositor resource.  */

struct wl_global *global_subcompositor;

static void
UnlinkSurfaceAction (SurfaceAction *subaction)
{
  subaction->last->next = subaction->next;
  subaction->next->last = subaction->last;
}

static void
HandleOtherSurfaceDestroyed (void *data)
{
  SurfaceAction *action;

  action = data;
  UnlinkSurfaceAction (action);
  XLFree (action);
}

static void
DestroySurfaceAction (SurfaceAction *subaction)
{
  XLSurfaceCancelRunOnFree (subaction->destroy_listener);
  UnlinkSurfaceAction (subaction);

  XLFree (subaction);
}

static Bool
CheckSiblingRelationship (Subsurface *subsurface, Surface *other)
{
  Subsurface *other_subsurface;

  if (other->role_type != SubsurfaceType
      /* The role might've been detached from the other surface.  */
      || !other->role)
    return False;

  other_subsurface = SubsurfaceFromRole (other->role);

  if (other_subsurface->parent != subsurface->parent)
    return False;

  return True;
}

static void
ParentBelow (View *parent, View *below, Surface *surface)
{
  ViewInsertBefore (parent, surface->view, below);
  ViewInsertBefore (parent, surface->under, surface->view);
}

static void
ParentAbove (View *parent, View *above, Surface *surface)
{
  ViewInsertAfter (parent, surface->under, above);
  ViewInsertAfter (parent, surface->view, surface->under);
}

static void
ParentStart (View *parent, Surface *surface)
{
  ViewInsert (parent, surface->under);
  ViewInsert (parent, surface->view);
}

static void
RunOneSurfaceAction (Subsurface *subsurface, SurfaceAction *subaction)
{
  View *target;

  if (!subsurface->role.surface || !subsurface->parent)
    return;

  if (subaction->type == PlaceAboveOther)
    {
      if (subaction->other != subsurface->parent
	  && !CheckSiblingRelationship (subsurface, subaction->other))
	/* The hierarchy changed in some unacceptable way between the
	   action being recorded and the commit of the parent.
	   Ignore.  */
	return;

      /* Determine the target under which to place the view.  If
         subaction->other is underneath the parent, then this will
         actually be subsurface->parent->under.  */
      target = ViewGetParent (subaction->other->view);

      /* After that, unparent the views.  */
      ViewUnparent (subsurface->role.surface->view);
      ViewUnparent (subsurface->role.surface->under);

      if (subaction->other == subsurface->parent)
	/* Re-insert this view at the beginning of the parent.  */
        ParentStart (subsurface->parent->view,
		     subsurface->role.surface);
      else
	/* Re-insert this view in front of the other surface.  */
        ParentAbove (target, subaction->other->view,
		     subsurface->role.surface);
    }
  else if (subaction->type == PlaceBelowOther)
    {
      if (subaction->other != subsurface->parent
	  && !CheckSiblingRelationship (subsurface, subaction->other))
	return;

      target = ViewGetParent (subaction->other->view);
      ViewUnparent (subsurface->role.surface->view);
      ViewUnparent (subsurface->role.surface->under);

      if (subaction->other != subsurface->parent)
	/* Re-insert this view before the other surface.  */
	ParentBelow (target, subaction->other->under,
		     subsurface->role.surface);
      else
	/* Re-insert this view below the parent surface.  */
	ParentStart (subsurface->parent->under,
		     subsurface->role.surface);
    }
}

static void
FreeSurfaceActions (SurfaceAction *first)
{
  SurfaceAction *action, *last;

  action = first->next;

  while (action != first)
    {
      last = action;
      action = action->next;

      DestroySurfaceAction (last);
    }
}

static void
FreeSubsurfaceData (void *data)
{
  SurfaceActionClientData *client;

  client = data;
  FreeSurfaceActions (&client->actions);
}

static SurfaceAction *
AddSurfaceAction (Subsurface *subsurface, Surface *other,
		  SurfaceActionType type)
{
  SurfaceAction *action;
  SurfaceActionClientData *client;

  action = XLMalloc (sizeof *action);
  action->subsurface = subsurface;
  action->type = type;
  action->other = other;
  action->destroy_listener
    = XLSurfaceRunOnFree (other, HandleOtherSurfaceDestroyed,
			  action);

  client = XLSurfaceGetClientData (subsurface->parent,
				   SubsurfaceData,
				   sizeof *client,
				   FreeSubsurfaceData);

  if (!client->actions.next)
    {
      /* Client is not yet initialized, so initialize the sentinel
	 node.  */

      client->actions.next = &client->actions;
      client->actions.last = &client->actions;
      client->actions.type = Sentinel;
    }

  action->next = client->actions.next;
  action->last = &client->actions;

  client->actions.next->last = action;
  client->actions.next = action;

  return action;
}

static void
RunSurfaceActions (SurfaceAction *first)
{
  SurfaceAction *action, *last;

  action = first->last;

  while (action != first)
    {
      last = action;
      /* Run the actions backwards so they appear in the right
	 order.  */
      action = action->last;

      RunOneSurfaceAction (last->subsurface, last);
      DestroySurfaceAction (last);
    }
}

static void
DestroySubsurface (struct wl_client *client, struct wl_resource *resource)
{
  wl_resource_destroy (resource);
}

static void
SetPosition (struct wl_client *client, struct wl_resource *resource,
	     int32_t x, int32_t y)
{
  Subsurface *subsurface;

  subsurface = wl_resource_get_user_data (resource);

  subsurface->pending_substate.x = x;
  subsurface->pending_substate.y = y;
  subsurface->pending_substate.flags |= PendingPosition;
}

static void
PlaceAbove (struct wl_client *client, struct wl_resource *resource,
	    struct wl_resource *surface_resource)
{
  Subsurface *subsurface;
  Surface *other;

  subsurface = wl_resource_get_user_data (resource);
  other = wl_resource_get_user_data (surface_resource);

  if (other != subsurface->parent
      && !CheckSiblingRelationship (subsurface, other))
    {
      wl_resource_post_error (resource, WL_SUBSURFACE_ERROR_BAD_SURFACE,
			      "surface is not a sibling or the parent");
      return;
    }

  AddSurfaceAction (subsurface, other, PlaceAboveOther);
}

static void
PlaceBelow (struct wl_client *client, struct wl_resource *resource,
	    struct wl_resource *surface_resource)
{
  Subsurface *subsurface;
  Surface *other;

  subsurface = wl_resource_get_user_data (resource);
  other = wl_resource_get_user_data (surface_resource);

  if (other != subsurface->parent
      || !CheckSiblingRelationship (subsurface, other))
    {
      wl_resource_post_error (resource, WL_SUBSURFACE_ERROR_BAD_SURFACE,
			      "surface is not a sibling or the parent");
      return;
    }

  AddSurfaceAction (subsurface, other, PlaceBelowOther);
}

static void
NoteDesyncChild (Surface *surface, Role *role)
{
  Subsurface *subsurface;

  subsurface = SubsurfaceFromRole (role);

  if (!subsurface->parent || !subsurface->parent->role
      || !subsurface->parent->role->funcs.note_desync_child)
    return;

  subsurface->parent->role->funcs.note_desync_child (subsurface->parent,
						     subsurface->parent->role);
}

static void
NoteChildSynced (Surface *surface, Role *role)
{
  Subsurface *subsurface;

  subsurface = SubsurfaceFromRole (role);

  if (!subsurface->parent || !subsurface->parent->role
      || !subsurface->parent->role->funcs.note_child_synced)
    return;

  subsurface->parent->role->funcs.note_child_synced (subsurface->parent,
						     subsurface->parent->role);
}

static void
SetSync (struct wl_client *client, struct wl_resource *resource)
{
  Subsurface *subsurface;

  subsurface = wl_resource_get_user_data (resource);

  if (subsurface->role.surface
      && !subsurface->synchronous)
    NoteChildSynced (subsurface->role.surface,
		     &subsurface->role);

  subsurface->synchronous = True;
}

static void
SetDesync (struct wl_client *client, struct wl_resource *resource)
{
  Subsurface *subsurface;

  subsurface = wl_resource_get_user_data (resource);

  if (subsurface->role.surface
      && subsurface->synchronous)
    NoteDesyncChild (subsurface->role.surface,
		     &subsurface->role);

  subsurface->synchronous = False;

  if (subsurface->pending_commit
      && subsurface->role.surface)
    XLCommitSurface (subsurface->role.surface, False);

  subsurface->pending_commit = False;
}

static const struct wl_subsurface_interface wl_subsurface_impl =
  {
    .destroy = DestroySubsurface,
    .set_position = SetPosition,
    .place_above = PlaceAbove,
    .place_below = PlaceBelow,
    .set_sync = SetSync,
    .set_desync = SetDesync,
  };

static void
DestroyBacking (Subsurface *subsurface)
{
  if (--subsurface->refcount)
    return;

  XLFree (subsurface);
}

static Bool
EarlyCommit (Surface *surface, Role *role)
{
  Subsurface *subsurface;

  subsurface = SubsurfaceFromRole (role);

  /* If the role is synchronous, don't commit until the parent
     commits.  */
  if (subsurface->synchronous)
    {
      subsurface->pending_commit = True;
      return False;
    }

  return True;
}

static void
MaybeUpdateOutputs (Subsurface *subsurface)
{
  int x, y, width, height, base_x, base_y;

  if (subsurface->role.surface->output_x == INT_MIN
      || subsurface->role.surface->output_y == INT_MIN)
    /* Valid base coordinates are not yet available.  */
    return;

  if (!subsurface->parent)
    /* A valid scale factor is not available.  */
    return;

  /* Compute the positions relative to the parent.  */
  x = subsurface->current_substate.x * subsurface->parent->factor;
  y = subsurface->current_substate.y * subsurface->parent->factor;

  /* And the base X and Y.  */
  base_x = subsurface->role.surface->output_x;
  base_y = subsurface->role.surface->output_y;

  /* Compute the absolute width and height of the surface
     contents.  */
  width = ViewWidth (subsurface->role.surface->view);
  height = ViewHeight (subsurface->role.surface->view);

  /* If nothing really changed, return.  */
  if (x == subsurface->output_x
      && y == subsurface->output_y
      && width == subsurface->output_width
      && height == subsurface->output_height)
    return;

  /* Otherwise, recompute the outputs this subsurface overlaps and
     record those values.  */
  subsurface->output_x = x;
  subsurface->output_y = y;
  subsurface->output_width = width;
  subsurface->output_height = height;

  /* Recompute overlaps.  */
  XLUpdateSurfaceOutputs (subsurface->role.surface,
			  x + base_x, y + base_y,
			  width, height);
}

static void
AfterParentCommit (Surface *surface, void *data)
{
  Subsurface *subsurface;

  subsurface = data;

  /* The surface might've been destroyed already.  */
  if (!subsurface->role.surface)
    return;

  /* Apply pending state.  */

  if (subsurface->pending_substate.flags & PendingPosition)
    {
      subsurface->current_substate.x
	= subsurface->pending_substate.x;
      subsurface->current_substate.y
	= subsurface->pending_substate.y;

      /* The X and Y coordinates here are also parent-local and must
	 be scaled by the global scale factor.  */

      ViewMove (subsurface->role.surface->view,
		subsurface->current_substate.x * subsurface->parent->factor,
		subsurface->current_substate.y * subsurface->parent->factor);
    }

  /* And any cached surface state too.  */
  if (subsurface->pending_commit)
    {
      XLCommitSurface (subsurface->role.surface, False);

      /* If the size changed, update the outputs this surface is in
	 the scanout area of.  */
      MaybeUpdateOutputs (subsurface);
    }
  subsurface->pending_commit = False;

  subsurface->pending_substate.flags = 0;
}

static Bool
Subframe (Surface *surface, Role *role)
{
  Subsurface *subsurface;

  subsurface = SubsurfaceFromRole (role);

  if (!subsurface->parent || !subsurface->parent->role
      || !subsurface->parent->role->funcs.subframe)
    return True;

  return subsurface->parent->role->funcs.subframe (subsurface->parent,
						   subsurface->parent->role);
}

static void
EndSubframe (Surface *surface, Role *role)
{
  Subsurface *subsurface;

  subsurface = SubsurfaceFromRole (role);

  if (!subsurface->parent || !subsurface->parent->role
      || !subsurface->parent->role->funcs.end_subframe)
    return;

  subsurface->parent->role->funcs.end_subframe (subsurface->parent,
						subsurface->parent->role);
}

static Window
GetWindow (Surface *surface, Role *role)
{
  Subsurface *subsurface;

  subsurface = SubsurfaceFromRole (role);

  if (!subsurface->parent || !subsurface->parent->role
      || !subsurface->parent->role->funcs.get_window)
    return None;

  return
    subsurface->parent->role->funcs.get_window (subsurface->parent,
						subsurface->parent->role);
}

static void
Commit (Surface *surface, Role *role)
{
  Subcompositor *subcompositor;
  Subsurface *subsurface;

  subcompositor = ViewGetSubcompositor (surface->view);
  subsurface = SubsurfaceFromRole (role);

  if (!subcompositor)
    return;

  /* If no buffer is attached, unmap the views.  */
  if (!surface->current_state.buffer)
    {
      ViewUnmap (surface->under);
      ViewUnmap (surface->view);
    }
  else
    /* Once a buffer is attached to the view, it is automatically
       mapped.  */
    ViewMap (surface->under);

  if (!subsurface->synchronous)
    {
      /* If the surface is asynchronous, draw this subframe now.
	 Otherwise it is synchronous, so we should wait for the
	 toplevel to end the frame.  */

      if (Subframe (surface, role))
	{
	  SubcompositorUpdate (subcompositor);
	  EndSubframe (surface, role);
	}

      /* If the size changed, update the outputs this surface is in
	 the scanout area of.  */
      MaybeUpdateOutputs (subsurface);
    }
}

static Bool
Setup (Surface *surface, Role *role)
{
  Subsurface *subsurface;
  View *parent_view;

  surface->role_type = SubsurfaceType;

  subsurface = SubsurfaceFromRole (role);

  subsurface->refcount++;
  subsurface->output_x = INT_MIN;
  subsurface->output_y = INT_MIN;
  role->surface = surface;
  parent_view = subsurface->parent->view;

  /* Set the subcompositor here.  If the role providing the
     subcompositor hasn't been attached to the parent, then when it is
     it will call ViewSetSubcompositor on the parent's view.  */
  ViewSetSubcompositor (surface->under,
			ViewGetSubcompositor (parent_view));
  ViewInsert (parent_view, surface->under);
  ViewSetSubcompositor (surface->view,
			ViewGetSubcompositor (parent_view));
  ViewInsert (parent_view, surface->view);

  /* Now add the subsurface to the parent's list of subsurfaces.  */
  subsurface->parent->subsurfaces
    = XLListPrepend (subsurface->parent->subsurfaces,
		     surface);

  return True;
}

static void
Rescale (Surface *surface, Role *role)
{
  Subsurface *subsurface;

  subsurface = SubsurfaceFromRole (role);

  /* The scale factor changed; move the subsurface to the new correct
     position.  */

  ViewMove (surface->view,
	    subsurface->current_substate.x * subsurface->parent->factor,
	    subsurface->current_substate.y * subsurface->parent->factor);
}

static void
ParentRescale (Surface *surface, Role *role)
{
  /* This is called when the scale factor of the parent changes.  */
  Rescale (surface, role);
}

static void
Teardown (Surface *surface, Role *role)
{
  Subsurface *subsurface;
  SurfaceActionClientData *client;
  SurfaceAction *action;
  Subcompositor *subcompositor;

  subsurface = SubsurfaceFromRole (role);

  /* If this subsurface is desynchronous, tell the toplevel parent
     that it is now gone.  */
  if (!subsurface->synchronous)
    NoteDesyncChild (role->surface, role);

  role->surface = NULL;

  if (subsurface->parent)
    {
      subcompositor = ViewGetSubcompositor (surface->view);

      ViewUnparent (surface->view);
      ViewSetSubcompositor (surface->view, NULL);
      ViewUnparent (surface->under);
      ViewSetSubcompositor (surface->under, NULL);

      client = subsurface->parent->client_data[SubsurfaceData];

      if (client)
	{
	  /* Free all subsurface actions involving this
	     subsurface.  */

	  action = client->actions.next;

	  while (action != &client->actions)
	    {
	      if (action->subsurface == subsurface)
		DestroySurfaceAction (action);
	    }
	}

      subsurface->parent->subsurfaces
	= XLListRemove (subsurface->parent->subsurfaces, surface);
      XLSurfaceCancelCommitCallback (subsurface->commit_callback);

      /* According to the spec, this removal should take effect
	 immediately.  */
      if (subcompositor
	  && Subframe (surface, role))
	{
	  SubcompositorUpdate (subcompositor);
	  EndSubframe (surface, role);
	}
    }

  DestroyBacking (subsurface);
}

static void
ReleaseBuffer (Surface *surface, Role *role, ExtBuffer *buffer)
{
  Subsurface *subsurface;

  subsurface = SubsurfaceFromRole (role);

  if (!subsurface->parent || !subsurface->parent->role)
    {
      XLReleaseBuffer (buffer);
      return;
    }

  subsurface->parent->role->funcs.release_buffer (subsurface->parent,
						  subsurface->parent->role,
						  buffer);
}

static void
HandleSubsurfaceResourceDestroy (struct wl_resource *resource)
{
  Subsurface *subsurface;

  subsurface = wl_resource_get_user_data (resource);
  DestroyBacking (subsurface);
}

static void
GetSubsurface (struct wl_client *client, struct wl_resource *resource,
	       uint32_t id, struct wl_resource *surface_resource,
	       struct wl_resource *parent_resource)
{
  Surface *surface, *parent;
  Subsurface *subsurface;

  surface = wl_resource_get_user_data (surface_resource);
  parent = wl_resource_get_user_data (parent_resource);

  /* If the surface already has a role, don't attach this subsurface.
     Likewise if the surface previously held some other role.  */

  if (surface->role || (surface->role_type != AnythingType
			&& surface->role_type != SubsurfaceType))
    {
      wl_resource_post_error (resource, WL_DISPLAY_ERROR_IMPLEMENTATION,
			      "trying to attach subsurface to surface with role");
      return;
    }

  subsurface = XLSafeMalloc (sizeof *subsurface);

  if (!subsurface)
    {
      wl_resource_post_no_memory (resource);
      return;
    }

  memset (subsurface, 0, sizeof *subsurface);
  subsurface->role.resource
    = wl_resource_create (client, &wl_subsurface_interface,
			  wl_resource_get_version (resource),
			  id);

  if (!subsurface->role.resource)
    {
      XLFree (subsurface);
      wl_resource_post_no_memory (resource);
      return;
    }

  wl_resource_set_implementation (subsurface->role.resource, &wl_subsurface_impl,
				  subsurface, HandleSubsurfaceResourceDestroy);

  /* Now the wl_resource holds a reference to the subsurface.  */
  subsurface->refcount++;

  subsurface->role.funcs.commit = Commit;
  subsurface->role.funcs.teardown = Teardown;
  subsurface->role.funcs.setup = Setup;
  subsurface->role.funcs.release_buffer = ReleaseBuffer;
  subsurface->role.funcs.subframe = Subframe;
  subsurface->role.funcs.end_subframe = EndSubframe;
  subsurface->role.funcs.early_commit = EarlyCommit;
  subsurface->role.funcs.get_window = GetWindow;
  subsurface->role.funcs.rescale = Rescale;
  subsurface->role.funcs.parent_rescale = ParentRescale;
  subsurface->role.funcs.note_child_synced = NoteChildSynced;
  subsurface->role.funcs.note_desync_child = NoteDesyncChild;

  subsurface->parent = parent;
  subsurface->commit_callback
    = XLSurfaceRunAtCommit (parent, AfterParentCommit, subsurface);
  subsurface->synchronous = True;

  if (!XLSurfaceAttachRole (surface, &subsurface->role))
    abort ();
}

static void
DestroySubcompositor (struct wl_client *client, struct wl_resource *resource)
{
  wl_resource_destroy (resource);
}

static const struct wl_subcompositor_interface wl_subcompositor_impl =
  {
    .destroy = DestroySubcompositor,
    .get_subsurface = GetSubsurface,
  };

static void
HandleBind (struct wl_client *client, void *data,
	    uint32_t version, uint32_t id)
{
  struct wl_resource *resource;

  resource = wl_resource_create (client, &wl_subcompositor_interface,
				 version, id);

  if (!resource)
    {
      wl_client_post_no_memory (client);
      return;
    }

  wl_resource_set_implementation (resource, &wl_subcompositor_impl,
				  NULL, NULL);
}

void
XLInitSubsurfaces (void)
{
  global_subcompositor
    = wl_global_create (compositor.wl_display,
			&wl_subcompositor_interface,
			1, NULL, HandleBind);
}

void
XLSubsurfaceParentDestroyed (Role *role)
{
  Subsurface *subsurface;

  subsurface = SubsurfaceFromRole (role);
  subsurface->parent = NULL;

  /* The callback is freed with the parent.  */
  subsurface->commit_callback = NULL;

  if (subsurface->role.surface)
    {
      ViewUnparent (subsurface->role.surface->view);
      ViewUnparent (subsurface->role.surface->under);
    }
}

void
XLSubsurfaceHandleParentCommit (Surface *parent)
{
  SurfaceActionClientData *client;

  client = parent->client_data[SubsurfaceData];

  if (client)
    RunSurfaceActions (&client->actions);
}

void
XLUpdateOutputsForChildren (Surface *parent, int base_x, int base_y)
{
  XLList *item;
  Subsurface *subsurface;
  Surface *child;
  int output_x, output_y, output_width, output_height;

  for (item = parent->subsurfaces; item; item = item->next)
    {
      child = item->data;
      subsurface = SubsurfaceFromRole (child->role);
      output_x = (subsurface->current_substate.x
		  * parent->factor);
      output_y = (subsurface->current_substate.y
		  * parent->factor);
      output_width = ViewWidth (child->view);
      output_height = ViewHeight (child->view);

      XLUpdateSurfaceOutputs (child,
			      base_x + output_x,
			      base_y + output_y,
			      output_width,
			      output_height);

      /* Record those values in the child.  */
      subsurface->output_x = output_x;
      subsurface->output_y = output_y;
      subsurface->output_width = output_width;
      subsurface->output_height = output_height;
    }
}

void
XLUpdateDesynchronousChildren (Surface *parent, int *n_children)
{
  XLList *item;
  Subsurface *subsurface;
  Surface *child;

  for (item = parent->subsurfaces; item; item = item->next)
    {
      child = item->data;
      subsurface = SubsurfaceFromRole (child->role);

      if (!subsurface->synchronous)
	/* The subsurface is desynchronous, so add it to the number of
	   desynchronous children.  */
	*n_children += 1;

      /* Update these numbers recursively as well.  */
      XLUpdateDesynchronousChildren (child, n_children);
    }
}